The thin slab continuous casting lines of iron mills have a heating furnace of the tunnel type disposed between casting equipment and rolling equipment. The heating furnace has inside thereof a large number of tire rollers arranged longitudinally thereof for transporting slabs as cast.
With reference to FIG. 1 showing a slab transport tire roller 1, the roller 1 comprises a tire 14 mounted on each of outer peripheral portions of a pipe 12 and is rotated by a rotating mechanism (not shown) connected to the roller. A cast slab 15 is transported as supported on the tires to the rolling equipment through a heating furnace 16 having a high temperature of at least about 1000.degree. C. When heated to a high temperature, the pipe 12 diminishes in rigidity and is liable to deflect or deform. To prevent this, cooling water is passed through the interior of the pipe 12, and the pipe 12 is covered with a heat-insulating material 18 over the portions thereof between the tires 14. The tire has a solid integral structure, an outer periphery which comes into direct contact with the slab of high temperature and an inner periphery which is held at about 250.degree. C. by the cooling effect of the pipe, so that a temperature difference of about 900.degree. C. occurs between the tire outer and inner peripheries.
The tire thermally expands on coming into contact with the hot slab. Owing to the diametrical temperature difference, the amount of thermal expansion is great toward the outer periphery adjacent to the slab and smaller toward the inner periphery close to the pipe than at the outer periphery. Consequently, the thermal stress occurring in the tire acts as a compressive stress at the outer peripheral side and as a tensile stress at the inner peripheral side. Cracks 20 therefore develop in the inner peripheral side where the stress is great, entailing the likelihood that the tire 14 will fracture as shown in FIG. 8.
Accordingly, a tire roller has been proposed which is improved in structure to overcome the drawback (JP-A-175581/1987). With reference to FIG. 9 showing the roller, a heat-insulating space 22 is formed between a tire 14 to be heated by contact with the slab and a pipe 12 to be cooled with water to reduce the heat loss of the material supported by the tire. The tire 14 has an inside diameter greater than the outside diameter of the pipe 12 and is fixed to the pipe 12 by a pair of annular hubs 24, 24 on opposite sides of the tire. The tire, annular hubs and pipe define a space serving as the heat-insulating space 22.
To transmit the rotational torque of the pipe 12 to the tire 14, the annular hubs 24, 24 are joined to both the tire and the pipe by welding over the entire circumferences thereof. Since heat transfer is effected between the tire 14 and the hubs 24 and between the pipe 12 and the hubs 24 through the welds 26, it has been impossible to obtain a sufficient heat-insulating effect.
The tire is heated by the slab, while the annular hubs are cooled by the pipe, with the result that a temperature difference occurs between the tire and the annular hubs. Consequently, a difference occurs between the tire and the hubs in the amount of thermal expansion, producing a stress between the tire and the hubs and leading to the likelihood of cracks developing.
An object of the present invention is to overcome the foregoing problems and to provide a slab transport tire roller comprising tires which are diminished in the thermal stress to be produced therein.